SEASON PREMIERE: This documentary, originally, was begun as a partnership venture with ESPN, but that other organization dropped out, leaving Frontline to finish and present this investigation on its own. It’s about the history and impact of head concussion injuries on pro football players – what happens medically, and how the NFL has reacted historically. It’s not unusual to describe any Frontline offering as a hard-hitting documentary – but this time it’s literal. Check local listings.